1. Field of the Invention
The present invention relates to a sequence control system and in particular to a sequence control system which allows programming of a sequence with the aid of a visible image of a sequence circuit diagram.
2. Description of the Prior Art
With the sequence control, a relay circuit is set up through a combination of relay elements and is destined to make logical decisions in a sequentially repeated manner. The sequence control of the old generation has been implemented in a "hard-wired" configuration. However, as the technologies of electronics have made significant progress lately, the logical decisions executed heretofore by the relay circuit tend to be performed through logical computation which is primarily executed through digital processing particularly with the aid of programmable computer referred to also as a programmable logic controller (PLC). One of the advantages of digital processing is derived from the fact that a control program can be software prepared with instruction codes to thereby facilitate various sequence controls in a flexible manner.
Japanese Laid-Open Patent Application No. 49-135097 laid open Dec. 26, 1974 corresponding to Koyamagi, et al., U.S. Pat. No. 3,944,987, issued Mar. 16, 1976 under the title "Sequence Control System" discloses a sequence control system of a digital logic circuit type in which a desired sequence instruction is read out from a sequence program unit and processing of the sequence is executed by an operation and control circuit. In this prior art system, a memory unit is employed for storing "ON" or "OFF" states of relay points constituted by branching points formed through interconnections of input contacts or relay contacts of output relays of a sequence circuit diagram and for additionally storing a level corresponding to the number of relay points which have been encountered before attaining a given relay point to which access is being made. With such arrangement, it is possible to process appropriately many and various sequence controls. However, the prior art sequence control system needs a memory unit of a great storage capacity for storing the "ON"/"OFF" states of the relay points as well as the level described above, involving great expense and difficulties in practical implementation.